80 Dr. H. Fol on the Family Tintinnodea. 
be understood if we assume the existence of a delicate mem= 
brane starting from this flexible border to become attached 
‘around the peristome. J have not seen this membrane; but 
its existence seems to me probable for the reasons indicated, 
and by analogy with the species in which the test, being 
more transparent, has allowed me to see a membrane at the 
spot indicated. 
In Cyttarocylis céstellula the margin of the test of adult 
specimens is also occupied by a prolongation which is less 
flexible than that of Codonella ventricosa, and is generally in- 
clined from without inwards (Pl. IV. fig. 8). It does not 
appear that this border can shut up completely; and therefore 
it will only act as a partial diaphragm. 
Together with certain common points of structure, the Tin- 
tinnodea therefore present a great variety in details, and we 
must expect in course of time to find a great diversity of forms. 
Thus, for a few days only, I met with a considerable number 
of specimens of a species which will be described further on, 
and which has the habit of attaching its agelutinant test to 
floating Algee, which the animal carries about with it. 
We know that certain forms which appear to be useful in 
the struggle for existence are often realized by very different 
animals and by very diverse means, although the final result 
may be very similar from the physiological point of view. 
The Ichthyosaur and the Cachalot, the Pterodactyle, the bird, 
and the bat are striking examples of this convergence of 
characters by adaptation. Among marine animals I have 
shown* that the Doliolum of the second generation, with its 
two kinds of buds, behaves like a Siphonophore, the zocecium 
being formed of a locomotive individual comparable to the 
bells of a Diphyes, and of feeding individuals or gasterozooids 
which nourish the whole colony. 
Another of these very frequent forms is that of very slender 
pelagic animals moved by palettes or cilia placed in the 
middle of their length. The larve of the Zoéa-form of cer- 
tain Decapod Crustacea are a well-known example of this 
animal form, which may very well be compared to the boats 
which are used in rowing matches. The great length has no 
injurious effect upon the rapidity of natation, just the con- 
trary ; but it renders all displacement which does not take 
place in the direction of the longitudinal axis very difficult. 
« . 5 . 7 
However, the animals which possess this external form have a 
* Société de Physique et d'Histoire naturelle de Genéve, “Sur la 
nutrition et la reproduction du genre Doliolum, communication faite en 
1875 ;” and “ Ueber die Schleimdriise oder den Endostyl der Tunicaten,” 
Morphol, Jahrb. Bd. i. p. 222 (1875). 2 
